[PATCH] cgroup: Fix potential overflow issue when checking max_depth

[PATCH] cgroup: Fix potential overflow issue when checking max_depth

[Xiu Jianfeng]

From: Xiu Jianfeng <xiujianfeng@huawei.com>

cgroup.max.depth is the maximum allowed descent depth below the current
cgroup. If the actual descent depth is equal or larger, an attempt to
create a new child cgroup will fail. However due to the cgroup->max_depth
is of int type and having the default value INT_MAX, the condition
'level > cgroup->max_depth' will never be satisfied, and it will cause
an overflow of the level after it reaches to INT_MAX.

Fix it by starting the level from 0 and using '>=' instead.

It's worth mentioning that this issue is unlikely to occur in reality,
as it's impossible to have a depth of INT_MAX hierarchy, but should be
be avoided logically.

Fixes: 1a926e0bbab8 ("cgroup: implement hierarchy limits")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
---
 kernel/cgroup/cgroup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 5886b95c6eae..044c7ba1cc48 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5789,7 +5789,7 @@ static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 {
 	struct cgroup *cgroup;
 	int ret = false;
-	int level = 1;
+	int level = 0;
 
 	lockdep_assert_held(&cgroup_mutex);
 
@@ -5797,7 +5797,7 @@ static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 		if (cgroup->nr_descendants >= cgroup->max_descendants)
 			goto fail;
 
-		if (level > cgroup->max_depth)
+		if (level >= cgroup->max_depth)
 			goto fail;
 
 		level++;
-- 
2.34.1

Re: [PATCH] cgroup: Fix potential overflow issue when checking max_depth

[Michal Koutný]

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Hello.

On Sat, Oct 12, 2024 at 07:22:46AM GMT, Xiu Jianfeng <xiujianfeng@huaweicloud.com> wrote:
> It's worth mentioning that this issue is unlikely to occur in reality,
> as it's impossible to have a depth of INT_MAX hierarchy, but should be
> be avoided logically.

Strictly speaking the overflow would be undefined behavior (Out of curiousity --
have you figured this out with a checker tool or code reading?)
Logically (neglecting UB), max_depth=INT_MAX would behave like intended
(no limit).

> Fixes: 1a926e0bbab8 ("cgroup: implement hierarchy limits")
> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
> ---
>  kernel/cgroup/cgroup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Michal Koutný <mkoutny@suse.com>

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Re: [PATCH] cgroup: Fix potential overflow issue when checking max_depth

[Xiu Jianfeng]

Hi Michal,

On 2024/10/14 21:37, Michal Koutný wrote:
> Hello.
> 
> On Sat, Oct 12, 2024 at 07:22:46AM GMT, Xiu Jianfeng <xiujianfeng@huaweicloud.com> wrote:
>> It's worth mentioning that this issue is unlikely to occur in reality,
>> as it's impossible to have a depth of INT_MAX hierarchy, but should be
>> be avoided logically.
> 
> Strictly speaking the overflow would be undefined behavior (Out of curiousity --
> have you figured this out with a checker tool or code reading?)

Thanks for your review, I figured it out with code reading.


> Logically (neglecting UB), max_depth=INT_MAX would behave like intended
> (no limit).

Agreed, this is the real 'no limit'.

> 
>> Fixes: 1a926e0bbab8 ("cgroup: implement hierarchy limits")
>> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
>> ---
>>  kernel/cgroup/cgroup.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Reviewed-by: Michal Koutný <mkoutny@suse.com>

Re: [PATCH] cgroup: Fix potential overflow issue when checking max_depth

[Tejun Heo]

On Sat, Oct 12, 2024 at 07:22:46AM +0000, Xiu Jianfeng wrote:
> From: Xiu Jianfeng <xiujianfeng@huawei.com>
> 
> cgroup.max.depth is the maximum allowed descent depth below the current
> cgroup. If the actual descent depth is equal or larger, an attempt to
> create a new child cgroup will fail. However due to the cgroup->max_depth
> is of int type and having the default value INT_MAX, the condition
> 'level > cgroup->max_depth' will never be satisfied, and it will cause
> an overflow of the level after it reaches to INT_MAX.
> 
> Fix it by starting the level from 0 and using '>=' instead.
> 
> It's worth mentioning that this issue is unlikely to occur in reality,
> as it's impossible to have a depth of INT_MAX hierarchy, but should be
> be avoided logically.
> 
> Fixes: 1a926e0bbab8 ("cgroup: implement hierarchy limits")
> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>

Applied to cgroup/for-6.12-fixes.

Thanks.

-- 
tejun